1. Field of the Invention
The present invention relates to a drum type-variable speed drive in which a shift fork is moved by rotation of a shift drum to shift gears of a transmission.
2. Description of Related Art
A drum type-variable speed drive is generally employed to shift gears of a transmission (see Japanese Patent No. 2515989, for example).
A drum type-variable speed drive disclosed in Japanese Patent No. 2515989 has a general structure, where, with rotation of a shift drum, a shift fork pivotally supported by a shift fork shaft in a slidable manner moves in the axial direction while being guided by a fork guide groove, whereby a shifter member of a transmission engaged with the shift fork is moved in order to drive the gear shift.
While the shifter member of the transmission is spline-fitted to a transmission shaft and capable of moving in the axial direction, it is incapable of rotating relative to the transmission shaft, and thus rotates integrally therewith.
A transmission gear pivotally supported by the transmission shaft in a rotatable manner is opposed to the shifter member, so that the components constitute a dog clutch.
When the shifter member moves by rotation of the shift drum through the shift fork, the dog clutch immediately meshes between the shifter member and the opposing transmission gear in some cases. In other cases, clutch teeth of the shifter member and the transmission gear abut on each other and do not immediately mesh, but meshes after their relative rotation.
For this reason, there is provided a so-called lost motion mechanism in which a spring for biasing the shifter member toward the transmission gear is installed while play is formed in the guide groove of the shift drum. Here, when the clutch teeth abut on each other, the shifter member is maintained in its position while being biased by the spring to wait for the meshing of the dog clutch. Then, when the dog clutch is ready to mesh, it is meshed at once by the spring load, and the gear is shifted.
Accordingly, since the biasing force of the lost motion spring constantly acts on the shift fork, an engaging pin portion of the shift fork slidably engaging with the guide groove of the shift drum is constantly pressed against one wall surface of the guide groove. This causes rotational friction on the shift drum, which increases the operation load at the time of shifting, and inhibits smooth shifting operations.